Capacitive electronic discs are molded plastic records which have television program information recorded in the surface thereof. A capacitive electronic disc, when played on a suitable player of the capacitive electronic pickup type, will provide television signal information which can be used to reproduce the programming recorded on the disc on a conventional television receiver.
Capacitive electronic discs are typically about 12 inches in diameter and have the television signal information recorded in a spiral information track which is formed in the playing surface of the disc. Although the capacitive electronic discs are somewhat similar in overall appearance to a conventional long-playing audio record, capacitive electronic discs are distinctly different with regard to the specifics of the actual recording made in the discs. There are, for example, about 10,000 information tracks per inch in a capacitive electronic disc, which is about 40 times greater than the groove density of a conventional audio record. Furthermore, the television signal information is recorded as vertical undulations in the base of the information track. The undulations of the capacitive electronic disc are about 1500 times smaller in amplitude and about 15 times smaller in wavelength than the corresponding signal information of a conventional audio record.
Because of the very small dimensions of the recorded signal elements of capacitive electronic discs, extreme care has to be taken to prevent any changes in the surface of the disc as these changes can adversely affect the quality of the television signals which are reproduced on playback of the discs. It has been found, for example, that defects only a few microns in size, which would have virtually no effect on playback of a conventional audio record, can cause major problems on playback of a capacitive electronic disc, such as the complete loss of the television signal.
Small defects in capacitive electronic disc which are, for example, about up to 5 microns in size, are referred to as microdefects to distinguish them from large defects such as dents, scratches and the like. Microdefects constitute one of the major causes for rejection of capacitive electronic discs and has proven to be one of the most difficult problems to overcome in commercial production. The microdefects in the molded disc, particularly if the microdefects are present in a high concentration in a given area, are believed to be a principle cause of a defect which is referred to as staining. A disc which is stained when examined in bright light will have dull areas or an overall hazy appearance instead of having a bright lustrous appearance. The appearance of the disc would seem to indicate to the unaided eye that the disc is stained on its surface with a foreign substance. However, more careful examination often shows that most of the areas that appeared stained on the surface of the disc are not contaminated with a foreign material as is a conventional stain, but rather the stains are in face mold impressions formed in the surface of the disc. Furthermore, the stained areas are often found to be areas which contained a high concentration of microdefects which have the effect of dulling optical reflection from the surface of the disc.
Many of the problems encountered with the microdefects and the related staining has been traced back to the production processes employed in the manufacture of discs. In the manufacture of capacitive electronic discs, the initial step is to record the desired television signal information on a magnetic tape. This tape is then used to drive a cutter head which cuts the desired signal information in the form of a spiral groove in a copper recording substrate. The copper recording substrate is then replicated by electroforming nickel on the recorded surface of the substrate to form a negative replica called a master. The master is then in turn, replicated by electroforming nickel on the surface of the master to form a series of positive replicas referred to as molds. The molds in turn are replicated by electroforming nickel onto the surface of the molds to form a series of parts which are referred to as stampers. The electroforming bath used in the replication process is of a modified nickel sulfamate type containing, in addition to the nickel sulfamate, various additives such as brighteners and the like, to ensure that the resulting electroformed parts are as conformal as possible with the surface of the parts on which they are electroformed.
The stampers are the parts which are actually used to press to capacitive electronic discs. The stampers prior to use are examined for defects, such as dents, microdefects and staining. Stampers which show any major defect or any significant staining, or the like, are either repaired prior to use or scrapped. The satisfactory stampers are then mounted on the molding platens of a record molding press. To mold the capacitive electronic discs a specially formulated electrically conductive plastic composition is molded under heat and pressure between a pair of the stampers. The information track which is in the stamper is reproduced in its mirror image in the molded disc. In addition to the desired information which is reproduced in the molded disc, however, any defects in the stampers are likewise reproduced in the surface of the molded disc.
Once the stamper is installed in the press and a number of discs are pressed from the record, it is often observed that a considerable amount of serious staining will start to occur on the stamper faces which is then replicated in the molded disc formed on the stained stampers. The staining which occured can broadly be classified in two groups; namely, additive staining and corrosive staining. Additive staining is generally caused by a foreign substance, such as water, contacting the surface of the stamper and leaving a residue when it drys. This type of staining can be removed with solvents or the like. However, the second and more serious type of staining that is observed is corrosive staining. Corrosive staining causes erosion of the surface of the stamper. The stamper when examined microscopically, can be seen to have a large number of microdefects in the surface of the stamper which tends to increase in size and depth as additional discs are molded with the stampers. The effects of corrosive staining cannot be repaired and eventually results in the stampers having to be scrapped.
The problem of corrosive staining of the stampers during molding is especially troublesome and difficult to overcome in that while all present stampers appeared to be subject to some degree of staining, certain batches of stampers have been found to have fair to good resistance to staining, while other batches of stampers almost immediately start staining once they are used to mold discs.
What would be highly desirable would be a capacitive electronic disc stamper which would have improved resistance to corrosive staining during the molding and also a method for consistently producing stampers having improved resistance to staining.